Why not just protect the wiring with the 80 or 90a breaker at the feed end, and run it right into the 100a breaker in the panel?
Probably because the garage is already fed, and the wire wont reach the house panel, and since these are SEC's feeding the garage it is most likely it's only a 3-wire feed, which if he goes to the house panel a 4-wire run will be required, and the fact that it will also add this load to the house 100 amp main?
I have been wiring out buildings and stand alone garages like this for a while because it saves allot of cost on installation and is the only code compliant method that still allows a 3-wire feed because the conductors are service entrance conductors not feeders, and you only need the disconnect at the garage since it is considered a service, and the wires never enter the house.
I would suggest changing the HL main to a 80-90 amp, or using a back fed 90 amp with a hold down clip in the garage panel.